1. Field of the Invention
The present invention relates to an electroacoustic transducer such as an electrodynamic loudspeaker, and more particularly to a diaphragm of a speaker.
2. Description of the Related Art
Referring to FIG. 1 of the accompanying drawings, illustrated is a partial cross sectional view of a conventional electrodynamic loudspeaker, which is one example of an electroacoustic transducer. The electrodynamic loudspeaker includes a pole yoke 1 projecting from a center portion of a back plate, and a magnet 2 placed around the pole yoke 1. A top plate 3 is placed on the magnet 2 such that a magnetic gap is created between the top plate 3 and pole yoke 1 to form a magnetic circuit. The top plate 3 is fixedly secured to a frame 5. A voice coil bobbin 4, having a voice coil 4a wound therearound, is positioned to oscillate in the magnetic gap and is supported by a damper 7. A cone-shaped diaphragm 8 is connected to the voice coil bobbin 4 at a center portion of the diaphragm, and a center cap 6 is provided at a truncated portion of the cone-shaped diaphragm 8. An upper opening periphery of the diaphragm 8 is supported by the frame 5 via an edge 9. A lead of the voice coil is connected to a terminal located on a lateral face of the frame 5 via a cable (Litz wire).
As described above, the electrodynamic speaker unit has the voice coil in the magnetic circuit, and causes the air to oscillate as an audio signal is input to the voice coil. An electromagnetic force generated according to Fleming's left hand rule activates the voice coil and in turn the diaphragm connected to the voice coil.
In general, the material of the speaker diaphragm should have a low density, large Young's modulus (high rigidity), certain internal loss and good environmental resistance. In recent times, attention has been given to fabricating a subwoofer (low tone or bass speaker) from a diaphragm made from a single resin material (raw material), and installation of the subwoofer in a vehicle.
A radiation efficiency of heat generated from the voice coil and transferred to a neck portion of the diaphragm is generally restricted (determined) by the material of the diaphragm, and improvement in the radiation efficiency depends upon physical characteristics of the resin material. Acoustic characteristics, of course, depend upon the physical characteristics of the resin material.
In addition, the diaphragm should be able to accept a large input and oscillate with a large amplitude if it is used for a subwoofer placed in a vehicle. In other words, the subwoofer is subjected to severe installation and operation conditions. Specifically, a great amount of current is supplied to the voice coil. Therefore, heat radiation efficiency should be improved and acoustic characteristics should be maintained.